JP5293879B2 - Exhaust device for internal combustion engine - Google Patents

Description

  The present invention relates to a technology for purifying exhaust gas from an internal combustion engine (engine).

Conventionally, there is a technique for reforming natural gas together with water vapor in the presence of a catalyst. For example, Patent Document 1 discloses a technique for reacting methane (CH ₄ ) with water vapor (H ₂ O) and converting it into a reformed fuel composed of carbon monoxide (CO) and hydrogen (H ₂ ). ing. Patent Document 2 discloses a technique for supplying gaseous fuel to an engine when the temperature of the engine is lower than a predetermined threshold when the engine is started.

JP-A-2005-030243 JP 2008-169704 A

According to the technique of Patent Document 1, CH ₄ can be converted into reformed fuel. However, in the case of Patent Document 1, it is necessary to supply water vapor separately. In Patent Document 1, the point of reducing the emissions by reforming CH _4, it is not any disclosure and suggestion.

The present invention has been made to solve the above problems, and an object thereof is to provide an exhaust device for an internal combustion engine capable of reducing the emissions by reforming CH _4.

In one aspect of the present invention, an exhaust system for an internal combustion engine includes an engine that can be operated by switching a plurality of types of fuel including CNG, an exhaust passage that communicates with the engine, and an exhaust passage provided on the exhaust passage. A reforming catalyst for reforming to produce a reducing agent; a NOx purification catalyst for purifying NOx by the reducing agent provided on the exhaust passage downstream of the reforming catalyst; and a temperature of the reforming catalyst. And control means for supplying CNG to the engine when the value is equal to or greater than a predetermined value .

The internal combustion engine exhaust device is mounted on a vehicle and includes an exhaust passage, a reforming catalyst, and a NOx purification catalyst. The exhaust passage communicates with the engine. The reforming catalyst is provided on the exhaust passage upstream of the NOx purification catalyst, and reforms CH ₄ to generate a reducing agent. The NOx purification catalyst purifies NOx with a reducing agent generated by the reforming catalyst. As described above, the exhaust device of the internal combustion engine reduces CH ₄ in the exhaust gas and reduces NOx by the reducing agent generated by reforming CH ₄ . Therefore, the exhaust device of the internal combustion engine can reduce emissions.

The engine uses CNG (Compressed Natural Gas) as fuel. In general, when CNG is used as a fuel, the emission amount of carbon dioxide (CO2), black smoke, and the like is small. On the other hand, in this case, the amount of CH4 discharged is relatively large. Therefore, in this aspect, the exhaust device of the internal combustion engine can generate a reducing agent based on CH4 discharged from the engine to purify NOx, and therefore can reduce emissions compared to other fuels.

Further, the engine can be operated by switching a plurality of types of fuel including CNG, and further includes control means for supplying CNG to the engine when the temperature of the reforming catalyst is equal to or higher than a predetermined value. In this aspect, the exhaust device for the internal combustion engine further includes control means. The engine is a bi-fuel engine that can be operated by switching a plurality of types of fuel including CNG. The control means is, for example, an ECU (Electronic Control Unit), and supplies CNG to the engine when the temperature of the reforming catalyst is equal to or higher than a predetermined value. Generally, when the temperature of the reforming catalyst is low, the conversion rate for reforming CH4 by the reforming catalyst becomes low. Therefore, in this aspect, the exhaust device of the internal combustion engine can reduce emissions by performing the CNG operation only when CH4 can be reformed.

In another aspect of the exhaust system for an internal combustion engine, the reforming catalyst reforms CH ₄ and H ₂ O to generate the reducing agent, and the predetermined value is a gas exhausted from the engine Of CH ₄ and H ₂ O. In this embodiment, the reforming catalyst reforms CH ₄ and H ₂ O. On the other hand, the target value of the conversion rate for reforming CH ₄ and H ₂ O differs depending on the CH ₄ concentration and H ₂ O concentration in the exhaust gas supplied from the engine. Therefore, in this aspect, the exhaust system of the internal combustion engine can expand the operating range using CNG by determining the above-described predetermined values based on the CH ₄ concentration and the H ₂ O concentration, thereby reducing emissions. be able to.

In another aspect of the exhaust system for an internal combustion engine, the reforming catalyst reforms CH ₄ and H ₂ O to generate the reducing agent, and the control means supplies CNG to the engine. In this case, a fuel having a higher ratio of H ₂ O concentration to CH ₄ concentration contained in the exhaust than CNG is additionally supplied to the engine for combustion. Here, “a fuel having a higher ratio of H ₂ O concentration to CH ₄ concentration contained in exhaust gas than CNG” corresponds to, for example, a fuel containing ethanol. As described above, the exhaust system of the internal combustion engine increases the water vapor supplied to the reforming catalyst by additionally supplying the fuel having a high H ₂ O ratio after combustion by the engine when using CNG, thereby The reforming reaction with the catalyst can be promoted. Therefore, the exhaust device of the internal combustion engine can improve the purification rate of CH _{4 in} the exhaust gas.

In another aspect of the exhaust apparatus for an internal combustion engine, the air conditioner further includes an air / fuel ratio sensor, the reforming catalyst reforms CH ₄ and H ₂ O to generate the reducing agent, and the air / fuel ratio sensor And provided on the exhaust passage downstream of the reforming catalyst. The reforming catalyst reforms H ₂ O together with CH ₄ . In addition, the air-fuel ratio sensor may be deteriorated by being exposed to water vapor. Therefore, the exhaust system of the internal combustion engine can suppress water exposure of the air-fuel ratio sensor by installing the air-fuel ratio sensor downstream of the reforming catalyst, and can reliably prevent damage to the air-fuel ratio sensor. it can.

It is an example of schematic structure of the exhaust apparatus of the internal combustion engine which concerns on 1st Embodiment. It is the figure which showed the structure of the front | former stage catalyst and the back | latter stage catalyst. And CH ₄ conversion, is an example of a graph showing the relationship between the exhaust gas temperature. It is a schematic block diagram of the exhaust apparatus of the internal combustion engine which concerns on a comparative example. It is an example of the flowchart which shows the process sequence of 1st Embodiment at the time of engine starting driving | operation. It is an example of the flowchart which shows the process sequence of 1st Embodiment at the time of the switch to CNG driving | operation. It is an example of schematic structure of the exhaust apparatus of the internal combustion engine which concerns on 2nd Embodiment. It is the figure which showed the flow of the exhaust gas in the exhaust apparatus of the internal combustion engine which concerns on 2nd Embodiment. It is an example of the graph of the time change of the condensed water generation amount after an engine start.

  Hereinafter, a first embodiment to a third embodiment, which are preferred embodiments of the present invention, will be described with reference to the drawings.

[First Embodiment]
(Outline configuration)
FIG. 1 is a schematic configuration diagram of an exhaust device 100 for an internal combustion engine according to the present invention.

  An exhaust device 100 for an internal combustion engine mainly includes an engine 1, an exhaust passage 2, a front catalyst 3, a rear catalyst 4, and an ECU 50.

  The engine 1 is a device that includes four cylinders 11 and generates power by burning a mixture of supplied fuel and air. Each cylinder 11 is supplied with fuel by a fuel supply valve (not shown) that supplies CNG and a fuel supply valve (not shown) that supplies liquid fuel. The liquid fuel is, for example, gasoline, light oil, alcohol such as methanol or ethanol, or a mixed fuel thereof. CNG and liquid fuel are respectively stored in a fuel tank (not shown).

  The engine 1 also includes a water jacket 13 serving as a coolant passage for cooling the cylinder 11 and the cylinder head 12 and the like. In the present embodiment, the water jacket 13 has a reduced wetted area with the exhaust port through which the exhaust gas passes and the surface of the exhaust manifold (hereinafter also simply referred to as “exhaust system”). That is, the water jacket 13 is designed so that the heat capacity of the exhaust system is reduced. Thereby, the exhaust device 100 of the internal combustion engine prevents water vapor from condensing in the exhaust system.

The exhaust gas generated by each cylinder 11 passes through the exhaust passage 2 via the exhaust port and the exhaust manifold. On the exhaust passage 2, a pre-stage catalyst 3 that is disposed immediately after the exhaust manifold and is a reforming catalyst of methane (CH ₄ ) and steam (H ₂ O), and a post-stage catalyst 4 that purifies NOx are provided. Yes.

Here, the front catalyst 3 and the rear catalyst 4 will be described in detail with reference to FIG. FIG. 2A is an example of a diagram illustrating the configuration of the front catalyst 3. As shown in FIG. 2 (a), the pre-catalyst 3 has a predetermined ratio of nickel (Ni) having a high CH ₄ reforming ability to an adsorbent having a high adsorption ability to CH ₄ which is the main component of CNG. It comprises mixed particles and aluminum oxide (Al ₂ O ₃ ). Here, the adsorbent is a mixture of silicon dioxide (SiO ₂ ) and Al ₂ O ₃ at a predetermined ratio. Thus, the front catalyst 3 adsorbed CH _4, purifying by reforming CH ₄ with water vapor. That is, the pre-stage catalyst 3 purifies CH ₄ by performing the reaction shown in the following reaction formula (1).

CH ₄ + H ₂ O → CO + 3H ₂ reaction formula (1)
Furthermore, the pre-stage catalyst 3 causes a partial oxidation reaction with oxygen (O ₂ ). That is, the pre-stage catalyst 3 causes the reaction shown in the following reaction formula (2) together with the reaction shown in the reaction formula (1).

CH ₄ + 1 / 2O ₂ → CO + 2H ₂ reaction formula (2)
As shown in the reaction formulas (1) and (2), the pre-catalyst 3 is reacted with carbon monoxide (CO) and hydrogen (reducing agents for the post-catalyst 4 by the reactions shown in the reaction formulas (1) and (2) ( H ₂ ). Thus, CO and H ₂ are examples of the “reducing agent” in the present invention. Thereby, as will be described later, the exhaust device 100 of the internal combustion engine can reform CH ₄ to reduce emissions. Further, the exhaust device 100 of the internal combustion engine can reduce the water vapor concentration of the exhaust gas after passing through the pre-stage catalyst 3 by reforming the water vapor in addition to CH ₄ .

  Preferably, the pre-stage catalyst 3 is an electrically heated catalyst (EHC) provided with a heater, and warm-up is executed based on control of the ECU 50.

The pre-stage catalyst 3 includes Ni as a metal exhibiting catalytic activity for the reforming of CH ₄ and water vapor, but instead includes rhodium (Rh), ruthenium (Ru), platinum (Pt), and the like. May be.

FIG. 2B is an example of a diagram illustrating a configuration of the rear catalyst 4. As shown in FIG. 2 (b), the post-catalyst 4 has particles in which an adsorbent (ZrO ₂ + CeO ₂ ) having a high NOx adsorption capability is provided with Rh having a high NOx reduction capability, and Al ₂ O _3. . Thereby, the rear catalyst 4 adsorbs NOx and reduces and purifies NOx. At this time, the post-catalyst 4 uses CO and H ₂ produced by the pre-catalyst 3 as nitrogen monoxide (NO) reducing agents. That is, the rear catalyst 4 causes the reactions shown in the following reaction formula (3) and reaction formula (4).

NO + H ₂ → 1 / 2N ₂ + H ₂ O Reaction formula (3)
2NO + CO → 1 / 2N ₂ + CO ₂ reaction formula (4)
As shown in the reaction formulas (3) and (4), the post-catalyst 4 can purify NOx and reduce emissions by using CO and H ₂ produced by the pre-catalyst 3 as a reducing agent. .

  Next, the configuration of the exhaust device 100 for an internal combustion engine will be described with reference to FIG. 1 again. The ECU 50 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like (not shown), and performs various controls on each component of the exhaust device 100 of the internal combustion engine. For example, the ECU 50 performs fuel injection control of liquid fuel and CNG based on the detection signal supplied as described above. The ECU 50 functions as control means in the present invention.

  In the following, “CNG operation” refers to an operation using CNG, and “liquid fuel operation” refers to an operation using liquid fuel.

(Control method)
Next, the control executed by the ECU 50 will be specifically described. Schematically, the ECU 50 determines that the temperature of the catalyst 3 (hereinafter referred to as “catalyst temperature T”) is less than a predetermined value (hereinafter referred to as “threshold T1”). CNG operation is prohibited until the threshold value T1 is reached. Thereby, the ECU 50 promotes the reforming reaction shown in the reaction formula (1), and realizes the improvement of the purification rate of CH ₄ and the reduction of the water vapor concentration after passing through the front catalyst 3.

This will be supplementarily described with reference to FIG. In the following, “CH ₄ conversion rate” refers to the proportion of CH ₄ reformed by the pre-stage catalyst 3 out of CH _{4 in} the exhaust gas discharged from the engine 1. The “H ₂ O conversion rate” refers to the ratio of H ₂ O reformed by the pre-stage catalyst 3 in the H ₂ O in the exhaust gas discharged from the engine 1. The “CH ₄ concentration” refers to the concentration of CH _{4 in} the exhaust gas, and the “H ₂ O concentration” refers to the concentration of H ₂ O in the exhaust gas.

FIG. 3 is an example of a map showing the relationship between the CH ₄ conversion rate and the exhaust gas temperature that is substantially equivalent to the catalyst temperature T. As shown in FIG. 3, the larger the exhaust gas temperature that is substantially equivalent to the catalyst temperature T, the more the reaction shown in the reaction formula (1) is promoted, and the CH ₄ conversion rate becomes higher. Similarly, as the exhaust gas temperature increases, the reaction shown in the reaction formula (1) is promoted, and as a result, the H ₂ O conversion rate increases. Therefore, the ECU 50 can reduce the CH ₄ concentration and the H ₂ O concentration after passing through the front catalyst 3 by performing CNG operation after the front catalyst 3 is warmed up. Further, the ECU 50 can supply H ₂ and CO that function as a reducing agent to the rear catalyst 4 by the reaction formula (1) and the reaction formula (2) by performing the CNG operation after the front catalyst 3 is warmed up. it can. Therefore, the ECU 50 can promote the reactions shown in the reaction formula (3) and the reaction formula (4) by performing the CNG operation after the pre-stage catalyst 3 is warmed up, and can reduce emission by NOx purification. .

Next, a method for determining the threshold value T1 will be specifically described. The ECU 50 determines the threshold T1 based on the CH ₄ concentration and the H ₂ O concentration of the exhaust gas before passing through the front catalyst 3. Specifically, the ECU 50 estimates the CH ₄ concentration and the H ₂ O concentration before passing through the front catalyst 3 when the CNG operation is started based on, for example, the fuel injection amount and the state of the engine 1. Then, the ECU 50 sets the threshold T1 with reference to, for example, a predetermined map or expression based on the CH ₄ concentration and the H ₂ O concentration. The above-described map and the like are created in advance based on, for example, experiments and stored in the memory of the ECU 50.

At this time, the ECU 50 preferably sets the threshold value T1 larger as the CH ₄ concentration and the H ₂ O concentration before passing through the front catalyst 3 are higher. For example, the ECU 50 sets the threshold value T1 to the temperature “T1b” shown in FIG. 3 when the CH ₄ concentration and the H ₂₀ concentration before passing through the front catalyst 3 are high. That is, in this case, since the CH ₄ concentration and H ₂ O concentration before passing through the front catalyst 3 are high, the ECU 50 suppresses the CH ₄ concentration and H ₂ 0 concentration after passing through the front catalyst 3 to a predetermined target value or less. Therefore, the threshold value T1 is set to a temperature T1b where the CH ₄ conversion rate and the H ₂ O conversion rate are high.

On the other hand, the ECU 50 sets the threshold value T1 to a temperature “T1a” lower than the temperature T1b when the CH ₄ concentration and the H ₂ O concentration before passing through the front catalyst 3 are lower than the above example. That is, in this case, ECU 50, since the front catalyst 3 passes CH ₄ concentration and _{H 2} O concentration before low, the CH ₄ conversion rate and _{H 2} O conversion rate, which corresponds to the temperature T1b CH ₄ conversion and the H _It is determined that it is not necessary to set up to ₂ O conversion rate. In other words, ECU 50, since it is already low CH ₄ concentration and _{H 2} O concentration in the front catalyst 3 before passing through time, if the catalyst temperature T1 is above temperature T1a, the front catalyst 3 after passing through CH ₄ concentration and H _It is determined that the ₂ O concentration can be suppressed below a predetermined target value. Therefore, ECU 50 in this case, the threshold value T1, CH ₄ conversion rate and _{H 2} O conversion rate is set at a lower temperature T1a than temperature T1b. Thus, when the CH ₄ concentration and the H ₂ O concentration before passing through the front catalyst 3 are low, the ECU 50 can start the CNG operation even when the catalyst temperature T is relatively low.

As described above, the ECU 50 determines the threshold value T1 based on the CH ₄ concentration and the H ₂ O concentration, thereby reducing the timing for limiting the CNG operation to the minimum while reducing the emission.

Next, a specific example when CNG operation is prohibited will be described. In the first example, the ECU 50 prohibits the CNG operation until the catalyst temperature T becomes equal to or higher than the threshold T1 when the catalyst temperature T is less than the threshold T1 when the engine 1 is requested to start the engine by CNG operation. The catalyst 3 is heated by a heater. Then, when the catalyst temperature T reaches the threshold value T1 or higher, the ECU 50 starts the engine 1 by CNG operation. In this way, the ECU 50 promotes the reaction shown in the reaction formula (1) in the front catalyst 3 during CNG operation to improve the CH ₄ purification rate, and promotes NOx purification in the rear catalyst 4. be able to. Further, the ECU 50 reduces the H ₂ O concentration after passing through the front catalyst 3 at the same time, thereby reducing the amount of condensed water generated in the exhaust passage 2 downstream from the front catalyst 3, and the sensor caused by the flooding. Deterioration such as breakage can be prevented.

In the second example, the ECU 50 prohibits switching to the CNG operation until the catalyst temperature T becomes equal to or higher than the threshold value T1 when the catalyst temperature T is lower than the threshold value T1 when requesting switching from the liquid fuel operation to the CNG operation. To do. Then, the ECU 50 performs the CNG operation when the catalyst temperature T becomes equal to or higher than the threshold value T1. Also by this, the ECU 50 can reduce the CH ₄ concentration to be discharged and can reduce the H ₂ O concentration after passing through the front catalyst 3.

(effect)
Next, the effect of the first embodiment will be supplementarily described.

Generally, when the H ₂ O concentration before passing through the front catalyst 3 is low, the reaction shown in the reaction formula (1) hardly occurs. Therefore, under conditions where water vapor condenses in the exhaust port, the exhaust manifold, etc., the H ₂ O concentration in the pre-stage catalyst 3 becomes low, and the reaction shown in the reaction formula (1) hardly occurs. Here, the condition for condensing water vapor corresponds to, for example, the case where the wall surface of the exhaust port is below the dew point. On the other hand, when the amount of fuel is increased to increase the air-fuel ratio in order to increase the H ₂ O concentration before passing through the front catalyst 3, CH ₄ also increases.

Considering the above, in the first embodiment, the ECU 50 performs the CNG operation when the catalyst temperature T is equal to or higher than the threshold value T1. Thus, the ECU 50 can promote the reaction shown in the reaction formula (1) without separately supplying water vapor to the front catalyst 3, and can reduce the CH ₄ concentration and the H ₂ O concentration after passing through the front catalyst 3. .

In the first embodiment, the exhaust device 100 for the internal combustion engine includes the rear catalyst 4 that is a NOx purification catalyst downstream of the front catalyst 3. Thereby, the exhaust device 100 of the internal combustion engine can promote the purification of NOx in the rear catalyst 4 based on H ₂ and CO generated by the reactions shown in the reaction formulas (1) and (2). That is, the ECU 50 can increase the reducing agent necessary for the reaction in the post-catalyst 4 by promoting the reforming reaction shown in the reaction formula (1), and can promote NOx purification.

  Furthermore, in the first embodiment, the exhaust device 100 for an internal combustion engine is designed so that the cooling area of the water jacket 13 for cooling the exhaust port and the exhaust manifold is small. This will be described using a comparative example shown in FIG. FIG. 4 is a schematic configuration diagram of an exhaust device 100x for an internal combustion engine according to a comparative example. The exhaust device 100x for an internal combustion engine mainly includes an engine 1x, an exhaust passage 2x, a front catalyst 3x, a rear catalyst 4x, and an ECU 50x.

  As shown in FIG. 4, in the comparative example, compared to the case of the first embodiment shown in FIG. 1, the water jacket 13x has an exhaust system for cooling the exhaust system communicating from each cylinder 11x to the exhaust passage 2x. The contact area is large. In other words, the exhaust device 100 for the internal combustion engine according to the first embodiment is less cooled to the exhaust system by the water jacket 13x than the exhaust device 100x for the internal combustion engine according to the comparative example. Therefore, in the first embodiment, the exhaust device 100 for an internal combustion engine can reduce the heat capacity of the exhaust system and prevent condensation of water vapor in the exhaust system as compared with the comparative example.

(Processing flow)
Next, the processing procedure in 1st Embodiment is demonstrated with reference to FIG. 5, FIG. In the following, first, the control at the start of the engine 1 will be described, and then the control at the time of switching from the liquid fuel operation to the CNG operation will be described.

1. FIG. 5 is an example of a flowchart showing the processing procedure of the first embodiment during the starting operation of the engine 1. The flowchart shown in FIG. 5 is repeatedly executed by the ECU 50 according to a predetermined cycle.

  First, the ECU 50 determines whether or not there is a request for starting the engine 1 (step S101). When it is determined that there is a start request (step S101; Yes), the ECU 50 determines whether there is a start prohibition request with liquid fuel (step S102). If the ECU 50 determines that there is a request to prohibit starting with liquid fuel (step S102; Yes), the ECU 50 executes the processing of steps S103 to S105.

  On the other hand, when the ECU 50 determines that there is no start request (step S101; No) or when it determines that there is no start prohibition request using liquid fuel (step S102; No), the process of the flowchart ends. Specifically, when the ECU 50 determines that there is no request to prohibit starting with liquid fuel (step S102; No), the ECU 50 performs fuel injection control using the liquid fuel and starts the engine 1.

Next, in step S103, the ECU 50 determines whether or not the catalyst temperature T is equal to or higher than a threshold value T1 (step S103). For example, the ECU 50 may estimate the catalyst temperature T based on a detection value of a temperature sensor (not shown) provided on the exhaust passage 2, and may calculate the catalyst temperature T based on a detection value of a temperature sensor installed in the upstream catalyst 3. You may measure. Further, the ECU 50 estimates the H ₂ O concentration and the CH ₄ concentration when the CNG operation is executed, and determines the threshold value T1 with reference to a predetermined map or the like.

When the ECU 50 determines that the catalyst temperature T is equal to or higher than the threshold value T1 (step S103; Yes), the ECU 50 starts the starting operation with CNG (step S104). That is, in this case, the ECU 50 considers the catalyst temperature T, the CH ₄ concentration, and the H ₂ O concentration, and determines that the CH ₄ concentration and the H ₂ O concentration after passing through the pre-stage catalyst 3 can be suppressed below a certain standard. . Then, ECU 50 in this case, by starting the start-up operation by CNG, it is possible to reduce the CH ₄ concentration and _{H 2} O concentration in the pre-stage catalyst 3 after passing through, _{H 2} and generated in the front catalyst 3 The post-catalyst 4 can purify NOx by making CO function as a reducing agent. That is, the ECU 50 can achieve low emission.

On the other hand, when the ECU 50 determines that the catalyst temperature T is not equal to or higher than the threshold value T1 (step S103; No), that is, when it is determined that the catalyst temperature T is lower than the threshold value T1, the ECU 50 prohibits the start operation at CNG. The front catalyst 3 is warmed up (step S104). For example, the ECU 50 warms up the front catalyst 3 with a heater installed in the front catalyst 3. And ECU50 continues the process of step S104 until the catalyst temperature T becomes more than threshold value T1. Thus, the ECU 50 suppresses emission reduction due to unpurified CH ₄ and suppresses damage to the sensor due to condensation of water vapor.

2. Control for Switching to CNG Operation FIG. 6 is an example of a flowchart illustrating a processing procedure of the first embodiment at the time of switching to CNG operation. The ECU 50 repeatedly executes the process of the flowchart shown in FIG. 6 according to a predetermined cycle.

  First, the ECU 50 determines whether or not the liquid fuel operation is in progress (step S201). If the ECU 50 determines that the liquid fuel operation is being performed (step S201; Yes), the ECU 50 determines whether there is a request for switching to the CNG operation (step S202). If the ECU 50 determines that there is a request for switching to CNG operation (step S202; Yes), the ECU 50 executes the processing of steps S203 to S205.

  On the other hand, when the ECU 50 determines that it is not the liquid fuel operation (step S201; No), or when it is determined that there is no request for switching to the CNG operation (step S202; No), the process of the flowchart is ended. Specifically, when the ECU 50 determines that it is not the liquid fuel operation (step S201; No), that is, in the case of the CNG operation, the CNG operation is continued. On the other hand, when there is no request for switching to the CNG operation (step S202; No), the ECU 50 continues the liquid fuel operation.

In step S203, the ECU 50 determines whether or not the catalyst temperature T is equal to or higher than the threshold value T1 (step S203). When the ECU 50 determines that the catalyst temperature is equal to or higher than the threshold value T1 (step S203; Yes), the ECU 50 starts a start operation with CNG (step S204). In this case, the ECU 50 can reduce the CH ₄ concentration and the H ₂ O concentration after passing through the front catalyst 3 by starting the starting operation with CNG, and at the rear stage based on H ₂ generated by the front stage catalyst 3. The catalyst 4 can purify NOx. That is, the ECU 50 can achieve low emission.

On the other hand, when the ECU 50 determines that the catalyst temperature T is not equal to or higher than the threshold T1 (step S203; No), the ECU 50 prohibits switching to the CNG operation and warms up the front catalyst 3 (step S205). And ECU50 continues the process of step S205 until the catalyst temperature T becomes more than threshold value T1. This also makes it possible for the ECU 50 to suppress a reduction in emissions due to unpurified CH ₄ and to prevent damage to the sensor due to condensation of water vapor.

[Second Embodiment]
Schematically, in the second embodiment, instead of or in addition to the first embodiment, the exhaust device 100 of the internal combustion engine uses an A / F sensor for detecting the air-fuel ratio in the exhaust gas as the front catalyst 3. Prepare downstream. Thereby, the exhaust device 100 of the internal combustion engine suppresses the damage of the A / F sensor due to the flooding. Hereinafter, after describing a schematic configuration of the second embodiment, a control method will be described.

(Outline configuration)
FIG. 7 is an example of a schematic configuration of an exhaust device 100 for an internal combustion engine according to the second embodiment. An exhaust system 100 for an internal combustion engine mainly includes an engine 1, an exhaust passage 2, a front catalyst 3, a rear catalyst 4, a switching valve 5, an A / F sensor 6, a bypass passage 7, an ECU 50, Is provided. Hereinafter, description of the same components as those in FIG. 1 will be omitted as appropriate.

  As shown in FIG. 7, the exhaust device 100 of the internal combustion engine communicates with the exhaust passage 2 upstream of the front catalyst 3 and the exhaust passage 2 downstream of the front catalyst 3 and upstream of the A / F sensor 6. A bypass passage 7 is provided. Further, a switching valve 5 for controlling the flow of exhaust gas is installed at a branch point between the exhaust passage 2 and the bypass passage 7 upstream of the front catalyst 3. The switching valve 5 supplies exhaust gas to the exhaust passage 2 in which the bypass passage 7 or the pre-stage catalyst 3 is installed based on the control signal S5 supplied from the ECU 50.

  An A / F sensor 6 for detecting an air-fuel ratio in the exhaust gas is installed on the exhaust passage 2 downstream of the bypass passage 7 and upstream of the rear catalyst 4. That is, the exhaust device 100 of the internal combustion engine includes the A / F sensor 6 in the exhaust passage 2 on the downstream side of the front catalyst 3.

(Control method)
Next, control executed by the ECU 50 in the second embodiment will be described. The ECU 50 performs a start operation with CNG having a high H / C ratio when starting the engine 1 in order to reduce emissions. In this case, during the CNG operation, the ECU 50 causes the steam reforming reaction shown in the reaction formula (1) by the pre-catalyst 3 to suppress the water coverage of the A / F sensor 6. This will be described with reference to FIGS.

FIG. 8A shows a state in the exhaust device 100 of the internal combustion engine during the CNG operation. The broken line arrows in FIG. 8A indicate the flow of exhaust gas. As shown in FIG. 8A, the ECU 50 controls the switching valve 5 so that the exhaust gas passes through the pre-stage catalyst 3 during the CNG operation. Thereby, the ECU 50 can promote the steam reforming reaction shown in the reaction formula (1) in the pre-catalyst 3 and reduce the H ₂ O concentration in the exhaust gas. Therefore, the exhaust device 100 of the internal combustion engine can reduce the H ₂ O concentration in the exhaust gas when passing through the A / F sensor 6, and can suppress the amount of water covered by the A / F sensor 6.

  FIG. 8B shows a state in the exhaust device 100 of the internal combustion engine when the liquid fuel operation is executed. The broken line arrows in FIG. 8B indicate the flow of exhaust gas. As shown in FIG. 8B, the ECU 50 controls the switching valve 5 so that the exhaust gas passes through the bypass passage 7 during the liquid fuel operation. That is, in this case, the ECU 50 does not supply the exhaust gas to the pre-catalyst 3 when the liquid fuel operation is less than the CNG operation. Thus, the ECU 50 uses the front catalyst 3 only during CNG operation, and suppresses deterioration of the front catalyst 3 and the like.

  The relationship between the region where the CNG operation is performed and the amount of condensed water generated will be described in more detail with reference to FIG. FIG. 9 is a graph of the change over time in the amount of condensed water generated after the engine 1 is started. As shown in FIG. 9, the amount of condensed water generated takes a maximum value when the engine 1 is started and then gradually decreases.

  First, until the time “t1” after the engine 1 is started, the ECU 50 performs CNG operation from the viewpoint of emission reduction. In this case, the ECU 50 controls the switching valve 5 so that the exhaust gas is supplied to the front catalyst 3. As a result, the ECU 50 causes the steam reforming reaction shown in the reaction formula (1) to occur in the pre-stage catalyst 3 in a predetermined period immediately after the start of the engine 1 where the amount of condensed water is large, and the water amount of the A / F sensor 6 is increased. Can be suppressed.

[Third Embodiment]
In the third embodiment, instead of or in addition to the first embodiment or the second embodiment, the ECU 50 performs the ratio of the H ₂ O concentration to the CH ₄ concentration contained in the exhaust gas after combustion during the CNG operation. (Hereinafter referred to as “H ₂ O / CH ₄ concentration ratio”) is additionally supplied with a fuel that is higher than CNG. Thereby, the ECU 50 increases the water vapor in the exhaust gas before passing through the front catalyst 3 to promote the reaction shown in the reaction formula (1).

This will be specifically described. First, the configuration of the exhaust device 100 for an internal combustion engine in the third embodiment will be described. In the third embodiment, in addition to the configuration of the first embodiment or the second embodiment, the exhaust device 100 for an internal combustion engine has a higher H ₂ O / CH ₄ concentration ratio in the exhaust gas that is additionally supplied during CNG operation than CNG. Fuel (hereinafter referred to as “high steam fuel”). Here, as the high steam fuel, for example, ethanol or a mixed fuel containing the same is applicable. The exhaust device 100 of the internal combustion engine may further store, for example, high steam fuel separately from the CNG and liquid fuel stored as the fuel of the engine 1 in the first embodiment or the second embodiment. In another example, the exhaust device 100 of the internal combustion engine may include a high steam fuel as the liquid fuel.

Next, control executed by the ECU 50 in the third embodiment will be described. The ECU 50 additionally supplies high steam fuel to each cylinder 11 during CNG operation. Thereby, the ECU 50 can increase the concentration of H ₂ O supplied from the engine 1 to the pre-stage catalyst 3 and promote the reaction of the reaction formula (1). Therefore, the ECU 50 can reduce the CH ₄ concentration in the exhaust gas after passing through the front catalyst 3 and realize low emission.

Preferably, the ECU 50 may set the threshold value T1 smaller in the third embodiment than in the first embodiment. Specifically, the ECU 50 may determine the threshold T1 in consideration of the supply amount of the high steam fuel in addition to the CH ₄ concentration and the H ₂ O concentration before passing through the front catalyst 3. For example, the ECU 50 stores, in advance, a map of the CH ₄ concentration, the H ₂ O concentration, the supply amount of the high steam fuel, and the threshold value T1 in a memory, and determines the threshold value T1 with reference to the map and the like. By doing in this way, ECU50 can reduce the period which prohibits CNG driving | operation, improving an emission.

DESCRIPTION OF SYMBOLS 1 Engine 2 Exhaust passage 3 Front stage catalyst 4 Rear stage catalyst 5 Switching valve 6 A / F sensor 7 Bypass passage 11 Cylinder 12 Cylinder head 13 Water jacket 50 ECU
100 Exhaust device for internal combustion engine

Claims (4)

An engine that can be operated by switching a plurality of types of fuel including CNG;
An exhaust passage communicating with the engine,
A reforming catalyst provided on the exhaust passage and reforming CH4 to generate a reducing agent;
A NOx purification catalyst that is provided on the exhaust passage downstream of the reforming catalyst and purifies NOx by the reducing agent;
Control means for supplying CNG to the engine when the temperature of the reforming catalyst is equal to or higher than a predetermined value;
An exhaust device for an internal combustion engine, comprising: The reforming catalyst reforms CH4 and H2O to generate the reducing agent,
The exhaust system for an internal combustion engine according to claim 1 , wherein the predetermined value is determined based on a CH4 concentration and a H2O concentration of gas exhausted from the engine. The reforming catalyst reforms CH4 and H2O to generate the reducing agent,
Wherein, when supplying CNG to the engine, an internal combustion according to fuel ratio of H2O concentration is high relative to CH4 concentration in the exhaust from the CNG, to claim 1 or 2 to burn in addition supplied to the engine Engine exhaust system. An air-fuel ratio sensor,
The reforming catalyst reforms CH4 and H2O to generate the reducing agent,
The exhaust device for an internal combustion engine according to any one of claims 1 to 3 , wherein the air-fuel ratio sensor is provided on the exhaust passage downstream of the reforming catalyst.

Images